Thunderstorms in the San Francisco Bay and Monterey Bay areas are expected to begin Thursday afternoon and continue through Friday morning. The chance of thunderstorms will diminish by late Friday morning, according to forecasters.

RAIN? IN AUGUST? Look at what the RAP computer model is showing for tonight in the SB. Lightning possible too. pic.twitter.com/ohcgFGRdzz

Weather service officials said they expect associated lightning strikes to be the most frequent in Monterey and San Benito counties initially, then spread north.

The thunderstorms will produce little or no rain and consequently increase the risk of wildfires. Isolated downbursts of wind may make fighting existing fires more difficult, forecasters said. The lightning would also pose risks to campers, hikers and people at outdoor sporting events, weather service officials said.

The high fire danger in the Bay Area has prompted the San Francisco Public Utilities Commission to close 23,000 acres of wildland on the Peninsula. The Peninsula Watershed -- which comprises the land around the Crystal Springs Reservoir, Pilarcitos Lake and San Andreas Lake -- was closed at noon Thursday and will remain closed Friday, according to the SFPUC.

The reservoirs help provide drinking water to 2.6 million SFPUC customers in San Francisco, Alameda, Santa Clara and San Mateo counties and the land around them has popular trails for hiking, running and biking. All trails in the area are closed through Friday and the SFPUC is limiting its own activities in the area as well. In addition the SFPUC is closely monitoring the 36,000-acre Alameda Watershed in the East Bay.

Massive wildfires have consumed thousands of acres of drought-ridden land in California in recent weeks, including nearly 70,000 acres in Lake, Yolo and Colusa counties and 8,000 acres in Napa and Solano counties.